There's this rss-to-activitypub server, if you really don't want to rely on a third-party service: https://github.com/dariusk/rss-to-activitypub
Or you can just use IFTTT (you can publish to Mastodon using the Webhooks trigger).
brunoamaral (mastodon.social)
@dinis in the end, I ended up using the API and #NodeRed to keep track of whatβs posted. @gregoryMS
nodered